1. Field of the Invention
The present invention relates to a device for sensing the load current for a control circuit of a H-bridge comprising MOS transistors and operating in class AB.
It applies for example to a control circuit of a voice coil used for positioning the actuator or the read/write heads of a control circuit of a disk on a desired track. The position of the heads is controlled through the current in the coil. The control of the movable coil uses a transconductance amplifier wherein the output current is proportional to the input voltage. Therefore, it is necessary to know the direction and the magnitude of the load current. The transconductance amplifier or control amplifier operates on the coil through a H-bridge, the coil being arranged in a diagonal of the bridge.
2. Discussion of the Related Art
Conventionally, in such a control circuit of a movable coil, the transconductance loop is closed through a sensing resistor serially connected with the load.
FIG. 1 shows a first example of a conventional circuit. The illustrated H-bridge includes four N-channel MOS transistors connected between a supply voltage Vdd and ground. The bridge is controlled by a control amplifier (not shown) connected to the gates of the four transistors. The drains of the two first transistors M1, M2, or high-side transistors, are connected to the supply voltage Vdd. Their respective sources are connected to the drains of two other transistors M3, M4, or low-side transistors, whose sources are grounded. The source of the first high-side transistor M1 is connected to a first terminal of a load 1 while the second terminal of the load 1 is connected through a sense resistor Rs to the source of the second high-side transistor M2. The two terminals of resistor Rs are connected to a sense amplifier (not shown) that transmits the voltage across resistor Rs to the control amplifier for controlling its gain. The load 1 corresponds for example to a resistor R serially connected with an inductor L.
FIG. 2 shows a second conventional control circuit of a moving coil. The difference with the circuit of FIG. 1 is that the load current sense resistor Rs is connected between the common sources of the two low-side transistors M3, M4 and ground. The terminals of resistor Rs are still connected to a sense amplifier.
In both circuits, the load current flows in the sense resistor Rs wherein a relatively high power dissipation occurs. Additionally, this causes a limitation of the load current for a given supply voltage Vdd.
A third conventional circuit is shown in FIG. 3. It includes three MOS transistors M1, M2, M3, an operational amplifier 2 and two mirror-connected NPN transistors T1, T2. Two high-side transistors M1, M2 have their respective drains connected to the supply voltage Vdd and their gates connected to a first terminal of the control amplifier. The source of transistor M1 is connected to the inverting input of the operational amplifier 2 and to the source of the low-side transistor M3. The output of the operational amplifier 2 is connected to the gate of transistor M3 whose source is connected to the collector of a first bipolar transistor T1. The two transistors T1, T2 have their bases connected with the collector of transistor T1 and their emitters grounded. The collector of transistor T2 is connected to the second terminal of the control amplifier. The non-inverting input of the operational amplifier 2 is connected to the source of the high-side transistor M2 and to a first terminal of load 1. The second terminal of load 1 is grounded. Such a circuit limits the power dissipated in as much as no serial resistor is used for sensing the load current. However, this circuit is operative only for class A control circuits where the current of the high-side power transistor M2 always flows in the load 1. Such a circuit cannot be used when the output is operated in class AB where the load current is different from the current flowing through the power transistor.
An object of the present invention is to avoid the above drawbacks of the existing sense circuits and to provide a load current sense circuit operable in class AB with a low dissipated power.